As wind turbine manufacturers strive to provide windfarm projects with more power and higher capacity factors, R&D on the various components that turbines comprise is vital. And nowhere is this more important than the gearbox – a crucial part of the turbine. As wind turbines get bigger and move offshore, this focus and attention to detail right along the supply chain will only intensify. And the stakes are huge, with demand for wind power exploding – from the oceans around the UK to the plains of Texas.

One fascinating technical conundrum being pondered at present is under which scenarios (if any) Direct Drive technology could usurp the conventional gearbox. To try and find the answer, fundamental research, development and testing is being performed in both camps.

Gearing up for improved efficiency

Speaking to Renewable Energy Focus at Husum WindEnergy, Prescott Logan, product manager at GE Drivetrain Technologies explained that the company is unique in combining expertise in both gearboxes and generators – unlike many other companies, which tend to specialise in only one of the areas. This, according to the company, has allowed the team to integrate planetary gearing with a medium speed generator into a system it calls IntegraDrive, resulting in a lighter, more compact, reliable and efficient system with fewer gears and bearings.

Robert Ciszak, advanced system leader, GE Drivetrain Technologies, explains “what we've done is taken a gearbox and removed the third stage of gears and bearings and directly connected the generator to the sun gear. [This gives] a number of benefits: it's smaller, lighter, more efficient, more compact, and, ultimately, more cost effective.”

Having an integrated system also takes away the problem of wind gusts causing a difference in structural deflection when hitting the blades, conflicting with the stationary generator. When the generator is right on centre and rigidly fixed to the gearbox, everything moves together, avoiding “unexpected” loads.

IntegraDrive is not the only integrated system on the market, but, according to GE Drivetrain Technologies, it stands out in part because it can be de-coupled in the tower – i.e. the gearbox or the generator can be accessed individually and be replaced without specialised cranes. In addition, the extra space created in the nacelle by the smaller system makes it possible to bring other equipment up into the tower to increase system efficiency, and perform more on-site service and maintenance.

Another benefit is that approximately 1.5% of energy is saved, says Ciszak. “1.5% on 2–3 MW is a lot of power!” The IntegraDrive is also about 2 tonnes lighter than a conventional drivetrain.

Logan points out that a smaller, more compact drivetrain has another practical advantage, in that it is easier to transport: “One of the challenges when you go above the 2 MW [size] is that logistically these components become very large. [But] let's say you can make a 3 MW drivetrain that is roughly the size of a conventional 1.5 MW drivetrain…you can [then] use conventional transportation.”

Asked why turbine designers should choose an integrated system over direct drive, Ciszak responds: “we've looked at studies, and we've done the curves many, many times, and over ranges from 1.5–8 MW, a geared solution is the right solution. I think automotive companies have shown that – every concept you see always include a gearbox, and they are very good at optimising for cost – that's the whole idea. I think the gear industry over time developed a bad name because, although it understood gears, it hadn't developed the ability to mass produce multi-megawatt sized components.”

With a background in engineering large drivetrains such as train locomotives, GE Drivetrain Technologies is well positioned to do just that – produce large gearboxes and generators in large volumes.

Responding to claims that geared wind turbines can be unreliable, the GE Drivetrain Technologies team points out that there are thousands of turbines running reliably every day, and that when things do go wrong, gearboxes are often landed with the blame, because problems in other parts of the system often manifest themselves in the gearbox.

Another reason for gearboxes' tainted reputation is that originally gearboxes and generators were not designed specifically for wind turbines.

Going forward

GE Drivetrain Technologies is already moving onto larger wind gearboxes designed for 2 and 2.5 MW turbines, and is planning to globalise its gearbox business. In the long term, the strategy is to move from being a gearbox component supplier to more of a system supplier.

“I think you'll see a constant stream of investment coming out of the company in the next 3–4 years. Product plans [are] laid out and we're going to keep bringing the market new technologies that advance the state-of-the art.” Logan says: “you'll see technologies that are broader than just the gearbox itself, really [addressing the whole system more] as GE Transportation [Drivetrain Technologies finds its] place in the wind power industry – as a system supplier as opposed to just a components supplier.”

However, despite its planned strategy, GE Drivetrain Technologies does not intend to become a wind turbine company – rather a technology supplier to wind turbine companies.

Siemens: the direct drive approach

Earlier this year, Siemens Energy announced the installation of two test wind turbines with direct drive (DD) technology at a test site in Denmark. The purpose of the project is to assess whether direct drive is competitive with geared machines for large turbines. A special focus will be placed on offshore applications.

According to Siemens, the main advantage of a direct drive turbine is that it removes the need for a gearbox. With fewer moving parts this has the potential to reduce maintenance costs, the logic goes. However, sources at Siemens acknowledge that direct drive does make the nacelle both heavier and more expensive in manufacturing.

The two turbines installed by Siemens will feature permanent magnet generators supplied by Siemens Industry Sector, and Converteam. The turbines themselves are similar in design to Siemens' SWT-3.6-107 offshore machine.

At the time of the announcement, Henrik Stiesdal, chief technology officer of the Siemens Wind Power Business Unit said, “direct drive wind turbines are an exciting technological option, but at this stage we cannot yet say whether they will prove to be a competitive alternative to geared turbines.”

Siemens believes that if – for large wind turbines – direct drive generators can, in serial production, become competitive with geared solutions, the simplicity and robustness of the direct drive technology will be a “decisive advantage” for offshore wind turbines.

In the first testing phase conducted by Siemens, several combinations of relevant technologies were tested, with a permanent magnet-excited synchronous generator design being the “winner”.

Permanent magnet generators require no excitation power, slip rings or excitation control system and the high field strength means they can also be made very compactly.

“Permanent magnets enable the mass of the direct drive generator to be reduced to approximately that of an equivalent gearbox plus conventional generator, plus a substantial 5% gain in overall efficiency,” Derek Grieve, technology director at Converteam says.

John F. Hill, manager renewables at Converteam, told Renewable Energy Focus that another reason people started looking at direct drive was the reliability which came with a “drastically reduced component count”. Converteam started with the development of a direct drive that could match the weight of the equivalent gearbox as part of a conventional power generator in 2002, and found that a permanent magnet solution was the simplest way of getting the weight down.

Just like GE Drivetrain Technologies, Converteam started with existing locomotive traction for railway, using permanent magnet motors: “We had technologies, but those technologies were only effective when they're operated at fairly high speeds and with fairly low powers, and so we had to extend the technology to achieve, first of all, very low speeds – torque transfer at the speed of a wind turbine rotor – at the same time as maximising cost-effectiveness and minimising weight,” Hill explains.

On Siemens' test turbines, the direct drive generator is supported by the main shaft rear end, and is positioned behind the mainframe and tower – where the gearbox traditionally sits.

According to Hill, the efficiency of permanent magnets is in the creation of magnetic fields, with low mass components, and without electrical exitation. The challenge is to capture those magnetic fields in a compact stator, and to transfer mechanical torque to them. As the rotor turns, the fields pass through the stator coils to create electrical power. The converter then accepts the natural power and converts it into direct current, which is then re-inverted into alternating current to match the grid.

Permanent magnetic generators are not new in themselves, but Hill says improvements have been made: “The big improvement that came with these machines above 2 MW was surface-mounted magnets. These are necessary because it's important to get the maximum effect from the magnets into the stator. If the magnets are embedded in the rotor, then a lot of the magnetic field is lost in that metal. So by surface-mounting them, more field can reach the stator.”

Why Direct Drive?

Siemens points out that the decision to test direct drive does not necessarily mean the company is changing its game: “Even though direct drive wind turbines are an exciting technological option, it cannot at this stage be concluded whether they will in the end prove to be a competitive alternative to geared turbines.”

According to Converteam's Hill, in these early years, the decision between direct drive and gearbox technology tends to be a generational one. Newcomers to the industry often want direct drive, whereas long-timers have a great deal of knowledge and investment associated with geared solutions, so take longer to invest in a new technology.

Integrated vs Direct Drive

According to GE engineer Ciszak, geared systems have several advantages relative to direct drive: “Direct drive has on-shore scalability issues above 1.7 MW using today's generator technology. The main limitation is transportation (infrastructure) and assembly crane lift limits. The geared solution provides onshore transportable solutions to 4 MW.”

Another point he raises is weight, saying direct drive generators trade weight for improved efficiency, as the most common method to increase generator efficiency is to increase copper and active material (magnet) mass, which has a negative effect on cost.

A geared solution allows generators to operate in the 250–400 rpm range. A compound planetary arrangement or a two-stage planetary drive runs at efficiencies between 98–99%, and the generators can be designed to run in the same range, giving a sub-system efficiency of around 97% for most of the useful power (speed) range.

According to Ciszak, wind loads and structural deflection can significantly change the operating air gap in a direct drive generator, and in a permanent magnet machine, this can significantly influence the bearing loads and reduce the generation efficiency. He says GE Drivetrain Technologies' IntegraDrive has a very controllable air gap and no structural deflections acting on the generator – thus raising the generation efficiency.

However, a geared solution also has its drawbacks. According to Siemens, the main drawback is the complexity of the gearbox. A typical gearbox for a large wind turbine has three phases, two planetary phases and one helical phase, with a total of 13 gears and pinions and 22 bearings.

With a direct drive, the need for the gearbox and subsequent maintenance is eliminated, reducing the complexity of the wind turbine. Moreover, according to Siemens, direct drive is “slightly more efficient than a gearbox-generator combination”.

But Siemens also acknowledges the challenges of the direct drive: “In the absence of the gearbox the generator has to be able to convert the full rotor torque. As a result the dimensions of a direct drive generator are very large compared with a high-speed generator with similar power rating, and a direct drive generator is typically much heavier than the combination of gearbox and high-speed generator for the same power rating.”

One thing is sure. The industry is eagerly awaiting the results from the Siemens direct drive trial, as it strives to move wind turbine technology to a new level.